1. Field of the Invention
This invention relates to liquid crystal cells, and more particularly to assemblies for holding the various components of a liquid crystal light valve together.
2. Description of the Related Art
The liquid crystal light valve (LCLV) is an optical image transducer capable of accepting a low-intensity light image and converting it, in real time, into an output image with light from another source. A general description of LCLVs is presented in Efron et al., "The Silicon Liquid-Crystal Light Valve", Journal of Applied Physics, Vol. 57, No. 4, Feb. 15, 1985, pages 1356-1368. LCLVs can be constructed to accept either visible or infrared input images, and can also operate from a direct electrical input through a charge coupled device (CCD) interface.
A generic LCLV is shown in FIG. 1. The device consists of a CdS or silicon photoconductor layer 10 and a nematic (normally transparent) liquid crystal layer 12 separated from the photoconductor 10 by a dielectric mirror 14 and a light blocking layer 16. The photoconductor layer 10 acts as an imaging, light-controlled voltage modulator for the liquid crystal layer 12. The broad-spectral band dielectric mirror 14 serves to reflect a modulated readout beam 18, while the light-blocking layer 16 prevents residual readout light from reaching the photoconductor layer 10. In response to an input light pattern 20, the impedance of photoconductor layer 10 lowers in proportion to the input light intensity at any particular spatial location. A voltage is applied across the assembly by means of a bias voltage source 22 connected to a transparent conductive electrode 24 and a transparent conductive counterelectrode 26, typically constructed from indium tin oxide (ITO). This voltage is switched to the liquid crystal layer 12 in proportion to the spatial reduction in impedance in photoconductor layer 10; the liquid crystal layer 12 and photoconductor 10 in essence act as a spatial voltage divider between the two electrodes. The application of the voltage pattern modulates the liquid crystal layer 12 to drive it above its electro-optic threshold in a pattern that replicates the input image intensity. Typical operating voltage levels are 10 volts rms at 10 kHz.
The LCLV assembly further includes transparent substrate face plates 28 and 30 for receiving the input and readout beams, respectively. Face plates 28 and 30 are constructed of optical glass flats or fiber optics, and sandwich the liquid crystal and associated light valve layers therebetween to provide coupling between input and output light.
The dimensions of the various LCLV components in FIG. 1 are greatly distorted for ease of illustration. In practice, the cumulative width of liquid crystal layer 12, dielectric mirror 14, light-blocking layer 16, photoconductor layer 10 and electrodes 24, 26 is much less than the widths of face plates 28,30.
A conventional mounting assembly for an LCLV is shown in cross-section in FIG. 2. The lower glass substrate 28 is supported in a well in an annular base 32, while the upper substrate 30 sits in a similar well in an annular retainer cap 34. The active elements of the LCLV are not shown individually since they are so much thinner than the glass substrates 28,30, but rather are indicated collectively by line 36.
The retainer cap 34 has a depending peripheral flange 38 that extends down to a level near the upper surface of base 32. The base and retainer cap are held together, clamping the glass substrates 28,30 and the active LCLV elements therebetween, by a number of bolts (not shown) that extend vertically through the depending cap flange 38 and the base 32.
The electrode 24, photoconductor 10, light-blocking layer 16 and dielectric mirror 14 of FIG. 1 are adhered to the upper surface of input face plate 28, while the counterelectrode 26 is adhered to the opposing lower face of the readout face plate 30, with the liquid crystal inbetween. Electrical leads 40 extend through an opening in the depending cap flange 38 to contact the LCLV electrode and counterelectrode, and are held in place by a retaining nut 42.
Four O-ring seals are required in the above structure. The first O-ring 44 is retained in a groove that extends around an inner upstanding flange 46 on base 32, to seal the space between the base 32 and retainer cap 34. Second and third O-rings 48 and 50 seal the abutment of base 32 and retainer cap 34 against shoulders on their corresponding face plates 28 and 30, while a fourth O-ring 52 seals the entrance for electrical lead 40.
A significant limitation of the above LCLV assembly is that it is not hermetically sealed, and requires a large inner cavity 54 to accommodate relative movement between the base and retainer cap. Oxygen can become trapped in the cavity and react with the liquid crystal, eventually destroying it.
An encapsulation technique that has been used in some liquid crystal applications, such as watches and calculators, uses a back-fill technique in which a small hole is formed in one of the substrates of a liquid crystal cell. The substrates are sealed together by a UV-cured epoxy with a space between them, and then subjected to a vacuum. Upon dipping the hole into liquid crystal and exposing the structure to atmospheric pressure, the liquid crystal is drawn through the hole to back-fill the space between the two substrates.
While this back-fill approach could be applicable to LCLVs, it has several important limitations. First, the space reserved for the liquid crystal is not adjustable. In an LCLV an ability to tune the thickness of the liquid crystal layer and thereby accommodate different readout wavelengths or correct for spacing errors, is highly desirable. Second, once the assembly has been completed it cannot be easily disassembled in case of a bad electrode. The expensive light valve substrates have to be forfeit along with the relatively inexpensive electrodes to disassemble the device. Third, some of the liquid crystal components tend to vaporize in a vacuum, so that the liquid crystal within the completed assembly may not have the same characteristics as it did before being subjected to a vacuum during the assembly process.